1. Field of the Invention
The present invention relates to an improved process for the preparation of hexene-1,6-dioic acid diesters. Such diesters are valuable intermediates that can facilely be hydrogenated into corresponding adipic acid diesters or adipates, which can then be hydrolyzed into adipic acid.
2 . Description of the Prior Art
Adipic acid, one of the starting materials for the production of nylon 66, is produced in vast amounts per annum. By reason of this fact alone, any new route for the synthesis of this diacid and/or derivatives thereof would be of fundamental interest to this art.
The preparation of a monoester of an alcohol and a butenoic acid by reacting carbon monoxide and an alcohol with an allyl alcohol monoester in the present of a palladium-based catalyst is described in U.S. Pat. No. 3,367,961. Thus, ethyl vinylacetate can be prepared from allyl acetate, carbon monoxide and ethanol in the presence of palladium chloride and palladium deposited on charcoal.
U.S. Pat. No. 4,611,082 indicates that the carbonylation of a solution of 1,4-diacetoxybut-2-ene in a aprotic, polar and non-basic solvent selected from among the nitriles, bis(2-methoxy)but-2-ene, bis(2-methoxyethyl) ether and methylene chloride, at 80.degree. to 140.degree. C., in the presence of a transition metal halide is virtually absent, and that, in the presence of an alcohol, the extent of carbonylation increases and is comparable to that observed for the carbonylation of but-2-ene-1,4-diol. With regard to this latter substrate, it is also indicated that satisfactory yields of straight-chain carbonylation compounds are not obtained under the above conditions. Thus, the preferred substrates are substituted in the 1,4-position by alkoxy groups.
As 1,4-diacetoxybut-2-ene is readily produced by acetoxylation of butadiene, a valuable addition to this art would be a process for preparing hex-3-ene-1,6-dioic acid diesters, in high efficiency, from 1,4-diacetoxybut-2-ene, in particular, and more generally from butenes disubstituted by acyloxy groups.
Published European Application No. 89/4,201,995 describes a process for the preparation of hex-3-ene dioic acid diesters by reacting carbon monoxide and an alcohol with at least one butene disubstituted by acyloxy groups in the presence of a palladium-based catalyst and a quaternary onium halide of a Group VB element of the Periodic Table selected from nitrogen and phosphorus, such element being tetracoordinated to carbon atoms, with the proviso that the nitrogen may be coordinated to two pentavalent phosphorus atoms, and the halide anion being a chloride or bromide.
The process of the '995 application, which provides appreciable results both in respect of its activity and in respect of its selectivity for a straight-chain dicarbonylated compound, however, presents the disadvantage or requiring the presence of at least one quaternary onium halide as indicated above. These onium compounds are promoters which are relatively expensive or not readily available and are susceptible to degradation over prolonged use.
Thus, need exists in this art for an alternate process that does not require a halogenated organic promoter, and particularly one wherein all or part of such organic promoter may be replaced by an inorganic halogenated promoter which is more readily available and relatively more stable over prolonged use.